System and method for measuring channel quality information in a communication system

Abstract

A system and method to measure channel quality in terms of signal to interference plus noise ratio for the transmission of coded signals over fading channels in a communication system. A Viterbi decoder metric for the Maximum Likelihood path is used as a channel quality measure for coherent and non-coherent transmission schemes. This Euclidean distance metric is filtered in order to smooth out short term variations. The filtered or averaged metric is a reliable channel quality measure which remains consistent across different coded modulation schemes speeds. The filtered metric is mapped to the signal to interference plus noise ratio per symbol using a threshold based scheme. Use of this implicit signal to interference plus noise ratio estimate is used for the mobile assisted handoff in a cellular system, power control and data rate adaptation in the transmitter.

What is claimed is:  
     
       1. A method for determining a signal to interference plus noise ratio, comprising the steps of: 
       establishing a set of path metrics corresponding to a set of predetermined signal to interference plus noise rations;  
       receiving a digital signal;  
       determining a path metric for said digital signal by establishing a set of signal to interference plus noise ratio values that correspond to a set of predetermined short term average of metric values and averaging a decoded path metric; and  
       mapping said path metric to said signal to interference plus noise ratio in said set of predetermined signal to interference plus noise ratios.  
     
     
       2. The method of claim  1 , wherein said digital signal is a coded signal. 
     
     
       3. The method of claim  1  wherein said digital signal is a trellis coded signal. 
     
     
       4. The method of claim  1  wherein the step of determining a path metric for said digital signal, further comprises the steps of: 
       establishing a set of signal to interference plus noise ratio values corresponding to a set of predetermined short term average of metric values, said short term average of metric values defined as M/μ;  
       determining a decoded path metric from said received digital signal using a decoder, said decoded path metric defined as m i ;  
       averaging m i ;  
       storing in a memory unit said average decoded path metric, said average decoded path metric defined as μ; and  
       determining an estimated Euclidean distance metric defined as M i .  
     
     
       5. The method of claim  4  wherein the step of determining the estimated Euclidean distance metric is performed using the following equation: 
       
         
           M i   =a M i-1 +(1- a ) m   l    
         
       
       Where said estimated Euclidean distance metric is defined as Mi and α is a predetermined filter coefficient which is greater than zero and less than 1.0.  
     
     
       6. The method of claim  5  including the steps of: 
       determining a standard deviation of M i ;  
       determining average metric thresholds defined as σ low  and σ high  based on said standard deviation of M i ;  
       determining a value for M i /μ by dividing said value of M i  by said value of μ;  
       mapping said value of M i /μ to a minimum value of said corresponding signal to interference plus noise ratio if M i /μ is less than σ low ;  
       mapping said value of M i /μ to a maximum value of said corresponding signal to interference plus noise ratio if M i /μ is greater than σ high ; and  
       mapping said value of M i /μ to said corresponding signal to interference plus noise ratio.  
     
     
       7. The method of claim  4  wherein said decoder is a Viterbi decoder for the maximum likelihood path. 
     
     
       8. A system for determining a signal to interference plus noise ratio, comprising: 
       means for establishing a set of path metrics corresponding to a set of predetermined signal to interference plus noise ratios;  
       means for receiving a digital signal;  
       means for determining a path metric for said digital signal by establishing a set of signal to interference plus noise ratio values that correspond to a set of predetermined short term average of metric values and averaging a decoded path metric; and  
       means for mapping said path metric to said signal to interference plus noise ratio in said set of predetermined signal to interference plus noise ratios.  
     
     
       9. The system of claim  8 , wherein said digital signal is a coded signal. 
     
     
       10. The system of claim  8  wherein said digital signal is a trellis coded signal. 
     
     
       11. The system of claim  8  wherein the means for determining a path metric for said digital signal, further comprises: 
       means for establishing a set of signal to interference plus noise ratio values corresponding to a set of predetermined short term average of metric values, said short term average of metric values defined as M i /μ;  
       means for determining a decoded path metric from said received digital signal using a decoder; said decoded path metric defined as m i ;  
       means for averaging m i ; and  
       means for storing in a second memory unit said average decoded path metric, said average decoded path metric defined as μ; and  
       means for determining an estimated Euclidean distance metric defined as M i .  
     
     
       12. The system of claim  11  wherein the means for determining the estimated Euclidean distance metric is performed using the following equation: 
       
         
           M i   =a M i-1 +(1- a ) m   l    
         
       
       where said estimated Euclidean distance metric is defined as M l  and α is a predetermined filter coefficient which is greater than zero and less than 1.0.  
     
     
       13. The system of claim  12  further comprising: 
       means for determining a standard deviation of M i ;  
       means for determining average metric thresholds defined as σ low  and σ high  based on said standard deviation of M i ;  
       means for determining a value for M i /μ by dividing said value of M i  by said value of μ;  
       means for mapping said value of M i /μ to a minimum value of said corresponding signal to interference plus noise ratio if M i /μ is less than σ low ;  
       means for mapping said value of M i /μ to a minimum value of said corresponding signal to interference plus noise ratio if M i /μ is less than σ high ; and  
       means for mapping said value of M i /μ to said corresponding signal to interference plus noise ratio.  
     
     
       14. The system of claim  11  wherein said decoder is a Viterbi decoder for the maximum likelihood path.